The life history of the gastropodThyonicola americanaTikasingh, endoparasitic in a seasonally eviscerating holothurian host

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Abstract
The endoparasitic gastropod Thyonicola americana Tikasingh attaches to the intestine of the dendrochirote holothurian Eupentacta quinquesemita (Selenka). Evisceration by E. quinquesemita is a seasonal event occurring in the autumn and as a result of their attachment to the viscera, T. americana is annually expelled from its host. The effect of evisceration on the life cycle of T. americana was investigated. Reproduction of the parasite is continuous, but the number of specimens at an advanced reproductive state was highest in summer and decreased coincident with seasonal evisceration. The parasites were not found attached to regenerating or newly regenerated viscera and there seemed to be a ‘seasonality’ in maturation imposed on T. americana by host seasonal evisceration. The optimum period for infection by T. americana is in the spring and the minimum length of the parasite's life cycle is approximately six months. Parasites expelled during evisceration perish, and so the majority of T. americana do not complete their life cycle. This may be compensated by the high fecundity of successful parasites and their success appears to be amplified by the high density of hosts for infection. The life cycle of T. americana is compared with that of entoconchid species parasitic in aspidochirote holothurians.